1. Field of the Invention
The present invention relates to a solid-state imaging element comprising a plurality of photoelectric conversion layers stacked on a semiconductor substrate having a signal reading circuit formed thereon and particularly to a photoelectric conversion layer-stacked solid-state imaging element comprising a light shielding layer provided interposed between the lowermost photoelectric conversion layer and the semiconductor substrate.
2. Description of the Related Art
A prototype example of photoelectric conversion layer-stacked solid-state imaging element is one disclosed in JP-A-58-103165. This solid-state imaging element comprises three light-sensitive layers stacked on a semiconductor substrate. In this arrangement, Red (R), green (G) and blue (B) electrical signals detected in the respective light-sensitive layer are read out by MOs circuit formed on the semiconductor substrate.
The solid-state imaging element having the aforementioned configuration was proposed in the past. Since then, CCD type image sensors or CMOS type image sensors comprising a numeral light-receiving portions (photodiode) integrated on the surface of a semiconductor substrate and various color filters of red (R), green (G) and blue (B) have shown a remarkable progress. The present technical trend is such that an image sensor having millions of light-receiving portions (pixels) integrated on one chip is incorporated in digital still cameras.
However, the technical progress of CCD type image sensors and CMOS type images has been almost limited. The size of the opening of one light-receiving portion is about 2 μm, which is close to the order of wavelength of incident light. Therefore, these types of image sensors face a problem of poor yield in production.
Further, the upper limit of the amount of photocharge that can be stacked in one fine light-receiving portion is as low as about 3,000 electrons, with which 256 gradations can be difficultly expressed completely. Therefore, it has been difficult to expect CCD type or CMOS type image sensor that outperforms the related art products from the standpoint of image quality or sensitivity.
As a solid-state imaging element that gives solution to these problems, a solid-state imaging element proposed in JP-A-58-103165 has been reviewed. Image sensors disclosed in Japanese Patent Application No. 3405099 and JP-A-2002-83946 have been newly proposed.
In the image sensor described in Japanese Patent Application No. 3405099, three photoelectric conversion layers having a ultraparticulate silicon having different diameters dispersed in a medium are stacked on a semiconductor substrate. The various photoelectric conversion layers generate electric signal according to the amount of red, green and blue lights received, respectively.
The image sensor described in JP-A-2002-83946 is similar to that of Japanese Patent Application No. 3405099. Three nanosilicon layers having different particle diameters are stacked on a semiconductor substrate. Red, green and blue electric signals detected by the respective nanosilicon layers are read out by storage diodes formed on the surface of the semiconductor substrate.
FIG. 5 is a diagrammatic sectional view of a two pixel portion of a related art photoelectric conversion layer-stacked solid-state imaging element. In FIG. 5, on the surface portion of a P-well layer 1 formed on an n-type silicon substrate are formed a high concentration impurity region 2 for storing red signal, an MOS circuit 3 for reading out red signal, a high concentration impurity region 4 for storing green signal, an MOS circuit 5 for reading out green signal, a high concentration impurity region 6 for storing blue signal and an MOS circuit 7 for reading out blue signal.
These MOS circuits 3, 5 and 7 are each formed by impurity regions for source and drain formed on the surface of the semiconductor substrate and a gate electrode formed via a gate insulating layer 8. On the top of the gate insulating layer 8 and the gate electrode is stacked an insulating layer 9 to level the surface thereof. On the insulating layer 9 is stacked a light shielding layer 10. In most cases, the light shielding layer 10 is formed a thin metal layer. Therefore, an insulating layer 11 is formed on the light shielding layer 10.
The signal charge stored in the high concentration impurity regions 2, 4 and 6 for storing color signal are read out by the MOS circuits 3, 5 and 7, respectively.
On the insulating layer 11 shown in FIG. 5 is formed a pixel electrode layer 12 defined every pixel. The pixel electrode 12 for each pixel is electrically conducted to the red signal storing high concentration impurity region 2 for each pixel via a columnar electrode 13. The contact electrode 13 is electrically insulated from the parts other than the pixel electrode layer 12 and the high concentration impurity region 2.
On the top of the various pixel electrode layers 12 is stacked one sheet of a red signal detecting photoelectric conversion layer 14 common to all the pixels. On the top of the photoelectric conversion layer 14 is formed one sheet of a transparent common electrode layer 15 common to all the pixels.
Similarly, on the top of the common electrode layer 15 is formed a transparent insulating layer 16 on the top of which a pixel electrode layer 17 defined every pixel is formed. The various pixel electrode layers 17 and the corresponding green signal storing high concentration impurity regions 4 are respectively conducted to each other via a columnar contact electrode 18. The contact electrode 18 is electrically insulated from the parts other than the pixel electrode layer 17 and the high concentration impurity region 4. On the top of the various pixel electrode layers 17 is formed one sheet of a green detecting photoelectric conversion layer 19 as in the photoelectric conversion layer 14. On the top of the photoelectric conversion layer 19 is formed a transparent common electrode layer 20.
On the top of the common electrode layer 20 is formed a transparent insulating layer 21 on the top of which a pixel electrode layer 22 defined every pixel is formed. The various pixel electrode layers 22 and the corresponding blue signal storing high concentration impurity regions 6 are respectively conducted to each other via a columnar contact electrode 26. The contact electrode 26 is electrically insulated from the parts other than the pixel electrode layer 22 and the high concentration impurity region 6. On the top of the pixel electrode layers 22 is stacked one sheet of a blue detecting photoelectric conversion layer 23 common to all the pixels. On the top of the photoelectric conversion layer 23 is formed a transparent common electrode layer 24. A transparent protective layer 25 is formed as an uppermost layer.
When light is incident on this solid-state imaging element, photocharge is excited in the photoelectric conversion layers 23, 19 and 14 according to the amount of incident blue, green and red lights, respectively. When a voltage is applied across the common electrode layers 24, 20 and 15 and the pixel electrode layers 22, 17 and 12, respectively, the respective photocharge flows into the high concentration impurity regions 2, 4 and 6 from which it is then read out as blue, green and red signals by the MOS circuits 3, 5 and 7, respectively.
Among the components of light 50 which is obliquely incident on the related art photoelectric conversion layer-stacked solid-state imaging element shown in FIG. 5, blue light is absorbed by the photoelectric conversion layer 23, green light is absorbed by the photoelectric conversion layer 19, red light is absorbed by the photoelectric conversion layer 14, and infrared light hits the light shielding layer 10. The infrared light is converted to heat by the light shielding layer 10 but is partly reflected by the light shielding layer 10.
Though depending on the material constituting the photoelectric conversion layers 14, 19 and 23 and other factors, visible light which has been left unabsorbed by the photoelectric conversion layers 14, 19 and 23, too, is reflected by the light shielding layer 10. The reflected light 51 comes back sequentially through the photoelectric conversion layers 14, 19 and 23 to generate photocharge again therein.
As shown in FIG. 5, when the obliquely incident light 50 is reflected in the vicinity of the pixel border, the pixel which generates signal charge by incident light 50 and the pixel which generates signal charge by reflected light 51 are different from each other, causing insufficient separation of signal charge by pixels. Further, problem of color mixing among pixels cannot be neglected.